I keep hearing sporadic reports like this. The big thing with CloudFlare is you are putting them as the first hop in reaching your site. So they are an additional point of failure. Of course, it's not a zero-sum game, they could also end up increasing your uptime overall, and in many cases I believe that's the case.
Particularly for relatively low volume sites which have a short burst in traffic on occasion, CloudFlare can keep those sites running during the peaks.
I think the most important thing is transparency and correct expectations. If they set clear expectations, and they are transparent about how well they are meeting them, then it just comes down to delivery.
I found their status dashboard here: https://www.cloudflare.com/system-status. Unfortunately it doesn't show much long-term historical performance, it would be nice to see 30 days even 180 days of performance history to really evaluate them.
regal, did you find that when you had downtime on your site that it was reported in their status dashboard, and that was an accurate depiction of the service they provided? I think the worst-case scenario is getting hit with unreported downtime, because that brings up all sorts of questions.